Fluctuations and some strain related interaction mechanisms in structural phase transitions

Materials which undergo structural phase transitions with bilinear coupling between the spontaneous strain (e ^sp _i ) and the thermodynamic order parameter (Q) of the type λ_i e ^sp _i Q show characteristic fluctuation pattern at T > T _c. Snapshots of such pattern show tweedlike structures with butterfly shaped structure factors. Some experimental fluctuation patterns are reviewed and compared with the theoretical predictions.

At T ≤ T_c , characteristic microstructures often include structural twinning. The relevant energy expressions for order/disorder systems are recalled and their contribution to thermodynamic fluctuations are described. It is anticipated that some of the physical mechanisms may play a role also in improper ferroelastics with coupling of the type λ_i e ^sp _i Q ² _k where Qk is a component of a degenerate order parameter Q = {Qk}.     

Blockage effects on viscous fluid flow and heat transfer past a magnetic obstacle in a duct

The effect of lateral walls on fluid flow and heat transfer is investigated when a fluid passes a magnetic obstacle. The blockage ratio β that represents the ratio between the width of external magnet M y and the spanwise width L y is employed to depict the effect. The finite volume method (FVM) based on the PISO algorithm is applied for the blockage ratios of 0.2, 0.3, and 0.4. The results show that the value of Strouhal number St increases as the blockage ratio β increases, and for small β , the variation of St is very small when the interaction parameter and Reynolds number are increasing. Moreover, the cross-stream mixing induced by the magnetic obstacle can enhance the wall-heat transfer and the maximum value of the overall heat transfer increment is about 50.5%.     

Rigid biological composite materials: Structural examples for biomimetic design

Biological hard tissues are composites of inorganics and biopolymers, and, therefore, represent hybrid systems. The inorganic components may be oxides (e.g., SiO₂, Fe₃O₄), carbonates (e.g., CaCO₃) sulfides (e.g., FeS, CdS), or others, mostly in crystalline forms but also occasionally in glassy forms. The biopolymer is often proteinaceous, but can also involve lipids and especially polysaccharides (e.g., chitin). These hybrid materials can be found in single celled organisms (such as bacteria and protozoa), invertebrates (such as mollusks), insects (such as beetles), and vertebrates (such as mammals). A common denominator of all hard tissues is that they are hierarchically structured from the nanometer scale to the microscale and the macroscale. It is these controlled structures that give biological hard tissues their unique and highly evolved functional properties. The engineering properties include mechanical, piezoelectric, optical, and magnetic. The hard tissues can be in the form of nanoparticles, spines, spicules, skeletons, and shells. The objective of this paper is to demonstrate mechanical aspects of some of these hard tissues, to discuss their structure-function relationships (with examples from the literature as well as from our research), and to reveal their potential utility in materials science and engineering applications.     

Differentially Trivial and Rigid Rings of Finite Rank

In this paper we study the rings of finite rank which have only trivial derivations (respectively endomorphisms). The main results are characterizations of these rings. This revised version was published online in June 2006 with corrections to the Cover Date.     

Computer modelling and geometric construction for four-point synthesis of 4R spherical linkages

In this paper, computer modelling and geometric construction of Burmester curve for synthesis of spherical mechanisms is presented. Rigid shell guidance in four specified positions on a sphere is performed by a 4R spherical linkage. The synthesis of such a linkage requires obtaining the associated Burmester curves on the reference sphere. Based on the Burmester theory and beside the computational and modelling abilities of the symbolic mathematical software namely Maple, an accurate as well as fast procedure for geometric construction of Burmester curve is developed. In the first part, the concepts of orientation and position on the sphere, pole of the motion and the related parameters are extended for modelling by the Maple. In the second part, the concepts of complementary axis quadrilateral and its imaginary motion, the center and circle axis cones and Burmester curve are derived. In the final part, using the prepared procedure and through a numerical example, a 4R spherical linkage for guiding an antenna to meet four specified postures in a three-dimensional working space is synthesized.     

障碍物结构对预混火焰压力发展的影响

本文针对不同的可燃气混合物,研究了由于障碍物引起的火焰加速而导致的管内压力上升现象。在实验中,使用了三种不同形状的障碍物,同时改变障碍物的间距W和阻塞比BR。结果表明,障碍物的存在对管内压力的影响是十分巨大的,即使是较小的阻塞面积,也能导致压力大幅度上升。在障碍物阻塞比BR=0.5、障碍物间距约等于管道内径(W/D≈1)时,管内的峰值压力达到最大值。本实验所用的三种障碍物产生的管内峰值压力并不相同,以园环型的障碍物产生的压力最高,这说明,存在某一种最优化的障碍物,在相同的阻塞比条件下,该障碍物能最大地提高管内的峰值压力。     

Spirane-bridged ansa η-cyclopentadienyl-η-fluorenyl-zirconocene precatalysts

Synthesis of rigid ansa-zirconocene precatalyst systems with a C₂-bridge embedded in a spirane scaffold is described. Fulvenes were key intermediates in regio- and stereoselective preparation of the appropriate spirane ligands. Substitution in the cyclopentadienyl group in the ligand was effected by fulvene methodology. The zirconocenes were active precatalysts for the polymerisation of propene when activated with MAO. The structure of the parent zirconocene dichloride has been verified by X-ray crystal analysis.     

高性能复合材料——分子复合材料的研究进展

分子复合材料是一种在分子水平上分散的刚/柔性聚合物增强复合材料,其将传统增强纤维与树脂宏观复合的概念扩展到分子水平的微观复合上,具有高强度、高模量的优点。作为高性能复合材料的一类,研究与开发分子复合材料对高性能复合材料的扩展与创新具有重要意义。本文对分子复合材料的组成、制备方法及其相容性的控制进行了介绍,并综述了国内外分子复合材料的研究现状。最后提出分子复合材料亟需解决的问题并对其发展前景进行了展望。     

Air shock wave interaction with an obstacle covered by porous material

The interaction between an air shock wave and a rigid wall covered by a porous screen is investigated numerically and experimentally. A mathematical two velocity with two stress tensors model is used for studying the wave processes in saturated porous media. The process of reflection of a step-type wave from a rigid wall covered with a porous layer is considered, the effect of the porous medium and wave parameters on the reflection is analyzed, and the numerical results are compared with the experimental data.Received: 30 July 2002, Accepted: 24 December 2002, Published online: 27 May 2003     

A conductive rigid spheroidal inclusion in a transversely isotropic piezoelectric body subjected to an axial pull

The exact axisymmetric solution is derived for an infinite transversely isotropic piezoelectric body containing an electrically conductive, rigid spheroidal inclusion under an axial pull. A simple general solution is employed in which three quasi-harmonic functions are involved and can be assumed in a closed form. The arbitrary constants are determined from the continuity conditions at the surface of the inclusion. The load-deflection and load-potential relations are derived, especially for two degenerated cases that are very important in the strength analysis of composite piezoelectric materials.